docs: def cast(self, topic, value):
          """ Cast a string to the value based on the datatype """
          datatype_key = topic.meta.get('datatype', 'none')
          result = self._datatypes[datatype_key].cast(topic, value)
          validate_dt = topic.meta.get('validate', None)
          if validate_dt:
              result = self._datatypes[validate_dt].cast(topic, result)
          return result